Molecular Plant-Microbe Interactions Editor's Picks

Jane Glazebrook, Editor-in-Chief

March 2015

Similar Effector Repertoires Between Pathogenic and Saprophytic Verticillium SpeciesView Article

Given the rapidly increasing capacity for sequencing, as well as the decreasing cost, comparing the genomes of multiple strains and species has become an effective approach for uncovering genomic features that potentially underpin specific traits or ecological niches. In the March Focus Issue on genomics, Seidl et al. compare the gene repertoire and genome organization of Verticilliumtricorpus with those of two pathogenic relatives: V. dahliae and V. alfalfae. These species encode similar putative effectors; thus, a key question is what functions these proteins confer.

February 2015

Transport of effector proteins from bacterial pathogens into host cells is crucial for pathogen virulence. In gram-negative bacteria, transport occurs through the specialized type III secretion system. In the February issue, Xiaofeng Zhou and colleagues demonstrate that the protein VrpA—an essential virulence factor for Xanthomonas spp.—is required for efficient protein secretion via the type III secretion system and that it localizes to the bacterial periplasmic space. These findings expand the mechanical understanding of this important secretory process in Xanthomonas spp.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

January 2015

The fungus Epichloë festucae is a mutualistic symbiont of the grass Lolium perenne. In the January issue, Becker et al. report on using a forward genetic screen and finding that mutations in the cell-wall integrity MAP kinase cascade cause symbiotic defects, including increased proliferation of hyphae, early senescence of the host, and colonization of the vasculature. These results show that the cell-wall integrity MAP kinase cascade plays a key role in control of the symbiotic development of the fungus.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

December 2014

Phytophthora species produce hundreds of effector molecules, so it has been difficult to determine which ones are important for virulence and what their activities are. In the December issue, Xiong et al. demonstrate that one of these effectors, which is conserved among Phytophthora species, interferes with RNA silencing systems in the host and substantially promotes virulence. In addition to demonstrating the function of an important Phytophthora effector, this work shows that host RNA silencing systems interfere with Phytophthora virulence.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

November 2014

A Xanthomonas pathogen of rice produces transcription activator-like (TAL) effectors that activate expression of SWEET sucrose exporters in the host, providing a carbon source for the pathogen. In the November issue, Cohn and colleagues present a comprehensive study showing that Xanthomonas pathogens of cassava also produce TAL effectors critical for virulence that activate expression of a sucrose and glucose exporter. This work provides a strategy for engineering Xanthomonas resistance in cassava, an important food crop.

October 2014

Although ectomycorrhizal symbioses are widespread and important for plant health in many ecosystems, little is known about the genetic requirements for fungal differentiation during mycorrhizal development. In the October issue, Doré and colleagues report T-DNA mutagenesis of the basidiomycete Hebeloma cylindrosporum, resulting in identification of the gene HcMycE1. Disruption of this gene blocks fungal ability to differentiate fungal sheath and Hartig net structures. Identifying HcMycE1 sets the stage for genetic dissection of the fungal side of ectomycorrhizal symbioses.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

September 2014

RNA silencing provides a potent defense against plant viruses; however, most viruses encode suppressor proteins that inhibit one or more steps in the silencing pathway. Several viral RNA-silencing suppressors inhibit Argonaute (AGO) 1, but until now, only inhibition of AGO1-mediated RNA degradation has been observed. In the September issue, Karran and Sanfaçon report that the coat protein of Tomato ringspot virus appears to protect viral RNA from AGO1-mediated inhibition of translation.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

Chinese translation courtesy of You Lu, University of Minnesota

August 2014

Endo-Arabinanase BcAra1 Is Novel Host-Specific Virulence Factor of Botrytis cinerea View ArticleThe fungal necrotroph Botrytis cinerea is known to require enzymes that degrade host cell walls for its virulence, but degradation of arabinan was not known to be important to the process. In comprehensive work published in the August issue, Nafisi et al. demonstrate that B. cinerea produces an arabinanase, that the encoding gene is induced during infection, and that the gene is required for full virulence on Arabidopsis thaliana. Interestingly, arabinanase is expressed at lower levels during infection of other hosts and in these cases is not required for virulence, indicating the pathogen's ability to adapt its mode of attack to a particular host.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

July 2014

Metabolic Environments and Genomic Features Associated with Pathogenic and Mutualistic Interactions Between Bacteria and PlantsView Article

Comparative analyses of microbial genomic sequences can highlight genomic traits that may function as hallmarks of specific microbial lifestyles. In the July issue, Karpinets and colleagues report on coupling comparative genomics with metabolic network models to identify genomic traits associated with pathogenic versus mutualistic interactions with plants. These genomic hallmarks, which include a putative secretion system in pathogens and a putative RuBisCO-based metabolic pathway in mutualists, were successful in predicting the pathogenic versus mutualistic interactions of 20 recently sequenced microbes.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

June 2014

Rab/GTPase proteins are known to play important roles in protein secretion in eukaryotes through regulation of vesicle trafficking at multiple steps. In the June issue, Zhang et al. study Bcsas1, a gene that encodes a Rab/GTPase in Botrytis cinerea, and their detailed characterization reveals that Bcsas1 is important for protein secretion. B. cinerea is known to rely on a variety of secreted proteins for virulence. Accordingly, mutants in which Bcsas1 is disrupted exhibit reduced secretion of plant cell wall degrading enzymes and greatly reduced virulence on several host plants.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

May 2014

Saliva from caterpillars has been shown to trigger defense responses in dicot plants. In the May issue, Chuang et al. show that saliva from the fall armyworm triggers defenses in a monocot: maize. The researchers used an elegant system that included caterpillars able and unable to secrete saliva, thus ensuring the biological relevance of the results. Feeding by intact caterpillars was found to induce defenses that inhibit caterpillar growth, whereas feeding by impaired caterpillars did not have this effect.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

April 2014

Melanin is required for the successful penetration of leaf surfaces by fungal appressoria, but the nature of this requirement is not entirely clear. In the April issue, Ludwig and colleagues use a melanin-deficient mutant of Colletotrichum graminicola to demonstrate that melanin is not needed for appressoria to generate turgor pressure and penetrate ultrathin synthetic membranes. Rather, melanin provides the cell-wall strength needed for penetration of intact leaves.

Spanish translation courtesy of Diana Trujillo, University of Minnesota

March 2014

Pathogen effectors are molecules that contribute to virulence through suppression of host defense responses and other activities. In the March Focus Issue on Translational Research, Vleeshouwers and Oliver describe successful strategies for exploiting pathogen effectors to breed more resistant crops. This approach of turning the pathogen's own weapons against it will likely have broad application. Reading this review is essential for anyone interested in improving disease resistance in crops.

February 2014

Plants recognize potentially pathogenic microbes using receptor-like kinases that bind conserved microbial molecules. One of these molecules, bacterial elongation factor TU (EF-Tu), was thought to be recognized only by Brassicaceae, which respond to an 18-amino acid epitope known as elf18. In this issue, Furukawa and colleagues show that rice also recognizes EF-Tu, but responds to a different epitope. EF-Tu is highly conserved and is one of the most abundant proteins in bacterial cells. Rice and the Brassicaceae apparently have taken different evolutionary paths to recognition of this molecule.

January 2014

Kinases and Signal Transduction in Tomato and Solanaceae SpeciesView Article

Protein kinases are critical components of plant immunity. In the January issue, Singh et al. report development of a comprehensive system for studying protein kinases of tomato. The system includes full-length expression clones, an improved protoplast transient expression system, and a protein chip for detection of proteins that interact with kinases. The authors demonstrate the power of this system by using it to discover a kinase targeted by an effector from the pathogen Pseudomonas syringae.